X-Video Motion Compensation

X-Video Motion Compensation (XvMC), is an extension of the X video extension (Xv) for the X Window System. The XvMC API allows video programs to offload portions of the video decoding process to the GPU video-hardware. In theory this process should also reduce bus bandwidth requirements. Currently, the supported portions to be offloaded by XvMC onto the GPU are motion compensation (mo comp) and inverse discrete cosine transform (iDCT) for MPEG-2 video. XvMC also supports offloading decoding of mo comp, iDCT, and VLD ("Variable-Length Decoding", more commonly known as "slice level acceleration") for not only MPEG-2 but also MPEG-4 ASP video on VIA Unichrome (S3 Graphics Chrome Series) hardware.

XvMC was the first UNIX equivalent of the Microsoft Windows DirectX Video Acceleration (DxVA) API. Mac OS X also includes MPEG-2 acceleration capabilities, but Apple has chosen not to expose that API for use outside their own DVD-Video player application.

Popular software applications known to take advantage of XvMC include MPlayer, MythTV, and xine.

Each hardware video GPU capable of XvMC video acceleration requires a X11 software device driver to enable these features.

There are currently three X11 Nvidia drivers available: a 2D-only open source but obfuscated driver maintained by Nvidia called nv, a proprietary binary driver by Nvidia, and an open source driver based on reverse engineering of the binary driver developed by the Linux community called Nouveau. Nouveau is not pursuing XvMC support, the 2D nv driver does not support XvMC, and the official proprietary binary driver by Nvidia only supports MPEG-2 offloading (mo comp and iDCT) on hardware up to and including the GeForce 7000 series.

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